Butterfly valves have for many years been manufactured by casting a rough valve body, machining the several required surfaces including the end faces, the valve seat where the disc engages and seals, the actuator flange face and fastener openings, and the stem bore. All of these must be highly accurate surfaces for proper smooth function and full closure of the valve. These machining operations are very costly and sometimes difficult to achieve, especially on large valves having a substantial weight.
Consequently, the inventor herein, in efforts to lower the cost of the valve, and to improve its manufacturing ease, conceived of an injection molding technique for valve components as set forth in U.S. Pat. Nos. 4,774,750 and 4,777,977. That injection molding technique involved pressures in the general range of about 8,000 to 10,000 pounds per square inch and up. The earliest attempts in this regard resulted in the grey iron valve body breaking up under the pressures involved. Yet it is desirable to employ grey iron castings for cost reasons, if possible. Later attempts involved complex techniques in efforts to balance the pressures across certain surfaces of the valve body so as to not destroy the valve body under the tremendous injection molding pressures. Although it was found possible to do this for certain valve components, this advantage was not enough to instigate commercial use of the technique. The cost of molds for the injection process of these prior patents was high, being in the range of about $70,000. Consequently, the older, standard manufacturing method involving several costly machining steps has largely prevailed.
The assignee herein has previously employed the technology set forth in U.S. Pat. No. 4,740,347 to commercially produce certain components of butterfly valves. This involves injection molding of rubber around valve stem bearings after mounting them on a previously machined replica of the valve and stem, and placing them in the unmachined (or rough machined) opening in the valve housings.
Still, whichever of these known commercial manufacturing processes was used, the cost of producing quality butterfly valves has been relatively high, with production requiring several complex operations.